A balanced digital subscriber loop comprising a twisted wire pair carries both differential and common mode currents induced by the signal and noise sources, respectively. In a perfectly balanced loop, the common mode currents will not interfere with the differential current (information signal). However, when bridge taps, poorly twisted cable, and so on, cause the circuit to be unbalanced, longitudinal current injected by external noise sources will be converted into differential current at the receiver and detected as noise. Such noise can lead to errors by introducing jitter in timing extraction circuits or by causing false pulse detection. In digital loops, common mode noise can be conveniently categorized into the following: Impulse noise, Radio Frequency Interference (RFI) and Crosstalk Noise. With the trend towards higher bit rates in the loops, radio interference, typically caused by radio stations in the vicinity and hence transmitting on certain frequencies with a relatively narrow bandwidth, is assuming greater significance.
When telephone subscriber loops operated at relatively low frequencies, perhaps 3,000 Hz. or 4,000 Hz., interference could be dealt with adequately by using twisted wire cable, which helps to cancel out any induced interference, and by means of hybrid transformers. With the introduction of VDSL (Very high speed Digital Subscriber Loops) and ADSL (Asymmetric Digital Subscriber Loops), the frequency of operation is approaching the radio frequency bands and the aforementioned balancing of the cable is no longer sufficient to reduce the interference. As a result, common mode noise increases.
Various techniques, other than such balancing, are known for reducing interference or noise in a communication channel. For example, U.S. Pat. No. 5,555,277, discloses a technique for cancelling common mode switching noise to achieve reduced error rates in a local area network. This technique involves gain controllers at both transmitter and receiver ends to maintain signal integrity during transmission. In addition, noise cancellation at the receiver is performed by generating primary and inverted copies of the received signal, amplifying both primary and inverted signals, and then summing them to cancel the induced common mode noise. This technique is not entirely satisfactory because it addresses common mode noise within the transceiver but not common mode noise in the transmission channel itself.
To mitigate common mode noise on a pair of signal conductors, the system disclosed in U.S. Pat. No. 3,705,365 issued Dec. 5, 1972 uses a two conductor shielded cable, a three-winding transformer, and a bipolar differential amplifier. The common mode signal from the cable shield is used to cancel the common mode noise using the third winding in the transformer. This technique is not entirely suitable for twisted wire transmission lines, such as telephone subscriber loops.
U.S. Pat. No. 4,283,604 discloses an electronic hybrid in the form of a current source circuit with common mode noise rejection for a two-wire transmission system. The circuit provides an electronic interface for coupling signals with a transmission line and operates to cancel or negate the effect of unwanted impedance on the line, thereby improving common mode impedance across the pair and enhancing the common mode rejection to noise ratio in the two-wire system. Like a conventional hybrid transformer, this electronic hybrid will not operate satisfactorily when the line is not balanced, especially when the line is a relatively long telephone subscriber loop.
European patent application number 0 453 213 A2 discloses a radio receiver in which an adaptive notch filtering approach is used to reduce interference in a radio frequency received signal carrying digital data at 2.4 kilobits per second using a 3 to 30 MHz r.f. carrier. The adaptive notch filter is implemented using frequency domain analysis of quantized data to detect interference by comparing the received signal frequency spectrum with a known spectrum template. Any frequency band with higher power than the reference template is considered to have interference. A programmable notch filter is then tuned to nullify the signal power in the respective frequency band. Unfortunately, the received signal in the selected band is cancelled along with the interference, resulting in an undesirable loss of signal information. A further disadvantage is that the technique also requires the interference rejection filter to be trained, which entails the transmission of a training sequence periodically between transmissions of the data stream, thus reducing overall transmission efficiency.
Hence, none of these various techniques constitutes a satisfactory way of reducing common mode noise in subscriber loops operating at the proposed very high speed levels of VDSL or ADSL. In T1E1.4/96-084 dated Apr. 18, 1996, and at a VDSL workshop at IEEE Globecom, Nov. 18, 1996 in London, England, John Cioffi and John Bingham proposed doing so by measuring the voltage between ground and the centre tap of the usual hybrid transformer at the end of the subscriber loop and extracting a signal representing common mode noise. Cioffi et al. proposed to filter this common mode noise signal using an adaptive wide band filter to provide a radio frequency noise estimate and subtract it from a differential signal obtained from the secondary of the hybrid transformer to produce an error signal for tuning the adaptive filter to reduce that error signal to zero. The circuit can only tune the filter when there are quiet periods in the received signal. This is not entirely satisfactory because it involves timing to ensure that the quiet periods are detected. Because noise patterns may change, the filter must be tuned frequently, which increases overhead, reducing the efficiency of the transmission. Also, the adaptive filter has to have a bandwidth at least as wide as the bandwidth of the received signal which, in the case of VDSL, might be from zero to about 10 Mz. Such a filter would be complex and expensive to make. Moreover, the arrangement might not work in places where a proper ground cannot be located, such as a rocky region.
An object of the present invention is to eliminate or at least mitigate the disadvantages of the foregoing known techniques and provide a noise suppression arrangement that is better adapted to the reduction of common mode noise in two-conductor communications channels, such as twisted wire subscriber loops.